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The term polyfunctional reagent is used in the speci- I APER figfigkcTmEfication and claims of this application to include formalde- John A.Harpham and Harry W. Turner, Wiimington,

DeL, assignors to Hercules Powder Company, Wilmington, Del., acorporation of Delaware No Drawing. Filed Mar. 28, 1962, Ser. No.183,611

Claims. (Cl. 162--1'l46) This invention relates to a method for theproduction of an improved paper and, more particularly, to a method formaking paper having increased porosity and absorbency as well as uniformformation, employing cross-linked pulp.

In the art of porous absorbent papers, the most important properties areporosity, absorbency and formation. Porosity determines the rate atwhich liquids or gases can be passed through the paper and is of obviousimportance for filtration purposes. Absorbency governs the rate at whichliquids are picked up by the paper and, in making saturated andimpregnated papers, it controls the rate of production of the paper.Both efficient filtration and uniform pick-up of impregnant require apaper of uniform formation.

It is known that although different pulps respond at somewhat differentrates to the beating operation in papermaking, they all respond in asimilar manner, that is, with increased beating of the pulp theresultant paper has lower porosity and absorbency but more uniformformation. Therefore, the papermaker must make a compromise between asmall amount of beating to maintain high sheet porosity and absorbencyand a lar e amount of heating to obtain uniform formation.

Likewise, it is well known how to prepare cross-linked celluloseapplicable to the present invention by reaction with the polyfunctionaland equivalent reagents disclosed herein. However, insofar as known,such cross-linked cellulose has not been employed in the manufacture ofpaper, much less in accordance with the present invention to give thesurprising results of markedly increased porosity and absorbency withoutsacrifice in uniformity of formation.

An object of this invention is an improved paper and method of preparingsame. A further object is a process of preparing a paper havingincreased porosity and absorbency without sacrifice in uniformity offormation. A still further object is such a process wherein the fibrousmaterial employed is cross-linked cellulose. Another object is such aprocess wherein the fibrous material employed is cellulose which hasbeen reacted with a polyfunctional reagent.

It has been found according to this invention that a marked increase inporosity and absorbency of paper without sacrifice in uniformity offormation can be obtained by employing cross-linked cellulose as a feedin the preparation of paper. The cross-linking must be carried out priorto the refining operation (i.e. prior to beating) in conventionalpaperrnaking processes, the type cross-linkage being an intrafiberchemical bonding. By intrafiber chemical bonding is meant that thecross-linking occurs between the cellulose molecules within any givenfiber instead of between cellulose molecules in different fibers(interfiber chemical bonding). The cross-linking is effected by reactingcellulose with formaldehyde or a reagent having at least two reactivefunctional groups.

hyde and compounds having at least two functional groups reactive withcellulose or cellulosic materials. By the term poly as used herein ismeant at least two. In addition to formaldehyde, these reagents comprisepolyhalides, e.g. 1,3-dichloroacetone; polyaldehydes, e.g. glyoxal;polyepoxides, e.g. epichlorohydrin; and the like. For the reasons morefully discussed and exemplified hereinafter, when formaldehyde or apolyaldehyde is the cross-linking reagent we have found it necessary (1)that the cross-linking temperature not exceed about 60 C., (2) that themineral acid used as cross-linking catalyst be a substantiallynonoxidizing acid, and (3) that substantially all of the residual acidbe removed from the cross-linked pulp before drying it.

The improved paper of this invention is clearly evident from theExamples l2l given below. Percent and ratio, as used in the examples andelsewhere herein, are on a weight basis. In all the Examples 1-21 thesamples were refined at 2.5 percent consistency and neutral pH in a 1%pound Valley laboratory beater for 1.5 and 2.0 hours with bedplate armloads of 4.5-6.0 kg, depending on the condition of the beaters tackle asjudged by running a standard pulp. The samples were handsheeted to a 40pound basis weight (24 X 36500 sheet ream) using Noble and Woodhandsheeting equipment. This operation involved forming the handsheetfrom a 0.025% slurry in the deckle box (closed white water system), wetpressing it between felts, and drum drying it on the screen atapproximately 116 C.

Handsheets were tested for water absorption (TAPPI standard methodT432m-45) and air resistance (TAPPI standard method T460m 49). Thesetests actually measure the reciprocal of the property being determined.Method T432m-45 measures the time (in seconds) for a 0.1-ml. drop ofwater to be absorbed into the sheet; thus, the longer the time, thelower is the absorbency of the sheet. Method T460m-49 measures the time(in seconds) for cc. of air under a constant pressure to pass through1.0 square inch of paper; thus, the longer the time, the lower is theporosity of the sheet. The uniformity of handsheet formation was judgedvisually in comparison to control handsheets.

EXAMPLES 1, 8 AND 15 Tables 1, 2 and 4 CONTROL A 600 gram sample ofmechanically cleaned raw second-cut cotton linters was purified in thelaboratory by conventional technique including alkali digestion underelevated temperature and pressure followed by bleaching, then washed,centrifuged and dried to give a yield of 85.6% pulp. The pulp was madeinto handsheets and tested in the manner described above. Additionaldata are given in Tables 1, 2 and 4 below.

EXAMPLE 2 Table 1 BLANK A 600 gram sample of raw second-cut cottonlinters was steeped in 10 parts of 8% aqueous sodium hydroxide for 20minutes at about room temperature, then centrifuged and tumbled in vacuoovernight at room temperature. This treatment was followed byconventional purification as in Examples 1, 8 and 15. The resultinglinters were washed, centrifuged and dried to give a yield of 84.5%pulp. The pulp was made into handsheets and tested in the mannerdescribed above. Additional data are given in Table 1 below.

EXAMPLES 3 AND 16 Tables 1 and 4 EPICHLOROHYDRIN A 100 pound sample ofrew second-cut cotton linters was steeped in 10 parts of 7.7% aqueoussodium hydroxide for 20 minutes at 38 C.43 C. and centrifuged. Thealkali cellulose was charged to the digester at 35 C. and 4 pounds ofepichlorohydrin added and the mixture tumbled 1.5 hours during whichtime the temperature was raised to 110 C. This treatment was followed byconventional purification as in Examples 1, 8 and 15. The resultingcross-linked purified pulp was made into handsheets and tested asdescribed above. Additional data appear in Tables 1 and 4 below.

EXAMPLES 4, 13 and 17 Tables 1, 3 and 4 EPICHLOROHYDRIN A 600 gramsample of mechanically cleaned raw secpulp was made into handsheets andtested as described above. Additional data appear in Tables 1 and 4below.

EXAMPLES 6 AND 12 Tables 1 and 3 CONTROL A 1 pound sample ofmechanically cleaned raw second-cut cotton linters which had beencommercially purified in the plant in a manner similar to the laboratorypurification in Examples 1, 8 and 15 was made into handsheets and testedin the manner described above. Additional data are given in Tables 1 and3 below.

EXAMPLE 7 Table 1 EPICHLOROHYDRIN Table 1 Cross-linking reagent BeatingWater Air resist- Ex. time, hr. absorpance,sec./ Formation comparedAmount, tion, see. 100 cc.lin. to control Kind percent on fiber 1 None(lab. purified control for Examples 2-5) None g2 1 2 None (caustic steepblank for Examples 3-5) None 1. 57 2.0 103 56 3 Epichicrohydrin 4 1.5 102 More uniform. 4 do... 15.5 1.5 4 0.2 Do. 2.0 8 0.8 Do. 51,3-dichloroacetone 10 1. 5 42 6 Equally uniform.

2.0 57 13 D0. 6 None (commercial purified control for Example 7) None 61a;

. 87 7 Epiehlorohydrin 5 1.5 1 More uniform.

ond-cut cotton linters was steeped in 10 parts of 8% so- EXAMPLE9 diumhydroxide for 20 minutes at room temperature and Table 2 thencentrifuged. 93 grams of epichlorohydrin was U CONTROL sprayed on theresulting alkali cellulose and the mixture tumbled in vacuo for 70 hoursat room temperature. This treatment was followed by conventionalpurification as in Examples 1, 8 and 15 to give a yield of 83.0% pulp.The resulting cross-linked pulp was made into handsheets and tested asdescribed above. Additional data appear in Tables 1, 3 and 4 below.

EXAMPLES 5 AND Tables 1 and 4 1,3-DICHLOROACETONE A 600 gram sample ofraw second-cut cotton linters wa steeped in 10 parts of 8% aqueoussodium hydroxide for 20 minutes at room temperature and centrifuged. Tothe alkali cellulose was added 60 grams of 1,3-dichloroacetone in 500ml. of aqueous isopropyl alcohol and the resulting mixture tumbled invacuo for 4 hours at room temperature. After purifying as described inExamples 1, 8 and 15, the 84.0% yield of cross-linked EXAMPLE 10 Table 2FORMALDEHYDE The conditions of Example 9 were repeated with 60 grams offormaldehyde added to the acidic acetone-linters. mixture beforerefluxing. After being washed and puritied, the resulting 79.5% yield ofcross-linked pulp was made into handsheets and tested as describedabove. Additional data appear in Table 2 below.

3,069,311 6 EXAMPLE 11 EXAMPLE 18 Table 2 Table 4 GLYOXAL SUBSTITUTION(GLYCIDOL) 5 The condltlons of Example were repeated, using 60 A 600gram sample of mechanically cleaned raw grams of glyoxal instead offormaldehyde. After being second-cut cotton linters was steeped in 10parts of 8% washed and purified, the resulting 81.5% yield ofcrossaqueous sodium hydroxide for minutes at room temlinked pulp Wasmade into handsheets and tested as deperature and then centrifuged. Tothe alkali cellulose scribed above. Additional data appear in Table 2below. was added 92.8 grams of glycidol and the resulting mix- TableZCross-linking reagent Beating Water Air resist- EX. time, hr.absorpance,sec./ Formation com- Kind Amount, tion,see. 100 cc./in.-pared to control percent on fiber 8 None (lab. purified control) None1.5 59 12 2.0 104 43 9 None (blank boilcdinacidicacetone)--. None 3 61i1 88 5 10 Formaldehyde 10 1.5 8 More uniform.

2.0 76 15 Do. 10 1.5 is 11 D0. 2.0 81 16 Do.

EXAMPLE 14 4 ture tumbled in vacuo for 4 hours at room temperature. 1After purification as described in Example 1, 8 and 15, Tab 8 3 the85.7% yield of substituted pulp was made into hand- CROSS-LINKEDFIBER-NONCROSSJJINKED sheets and tested as described above. Additionaldata FIBER BLEND appear in Table 4 below.

A 500 gram sample of mechanically cleaned second- EX MPLE 19 cut rawcotton linters was steeped in 10 parts of 8% Table 4 n aqueous sodiumhydroxide 101 20 minutes at room tem SUBSTITUTION (a CHLOR0HYDRIN)perature and then centrifuged. 75 grams of epichlorohydrin was sprayedon the resulting alkali cellulose and A 560 gram f p 0f mfichafiltfallyCleaned raw the mixture tumbled in vacuo f 5 hours at mom second-cutcotton linters was steeped 111 10 parts of 8% 1 h perature. Thistreatment Was followed by conventional aqueous Sodlum q? for 2Q mmutesPurification as in Examples 1, 3 and 15 to give a yield i g i f l g- 50granis t f;- of 76.0% cross-linked pulp. 0.5 pound of this crossm fscum? one raked ul Wa bknded with O 5 Ound of mmerc. H alkali celluloseand the resulting mixture tumbled 1n t c h t P E Co 2 vacuo for 21 hoursat room temperature. After purificacot on i SIS Ott ype LS6 m es an tionas described in Examples 1, 8 and 15, the 77.6% resultng blend 9 pulpwas m hand yield of substituted pulp was made into handsheets and sheetsand tested as described above. Additional data testgd as described aboveAdditional data appear in appear in Table 3 below. Table 4 below blEXAMPLE 21 e 3 Table 4 MIXTURE 0F ortoss-LlNgtgInPsaND NONCROSS-LINKEDSUBSTITUTION (CHLOROACETONE) A 500 gram sample of mechanically cleanedraw Com Crow 36% Water Airw second-cut cotton linters was steeped in 10parts of 8% w ln l in s s ps stan rmati n maqueous sodium hydroxide for20 minutes at room tem- 5%? i Egg? pared) perature and then centrifuged.50 grams of chloroacetone in a 50% aqueous solution was sprayed onto thealkali cellulose and the resulting mixture tumbled in 1 None 2% vacuofor 16 hours at room temperature. After purifica- 13 None 1 1:5 4 Moreuniform, tion as described in Examples 1, 8 and 15, the 81.3% M as 5 g g2 g 88' yield of substituted pulp was made into handsheets and 2.0 36 12Do. tested as described above. Additional data appear in Table 4 below.

Table 4 HANDSHEET PROPERTIES OF CHEMICALLY MODIFIED LINTERS PULPSEFFECTOF CROSS-LTNKING VS. SUBSTITUTION Treatment Air re- Beating Watersistance, Example time, hr. absorpsec./l Reagent Percent Reaction tion,see. ccJin,

on fiber None (lab. purified control for None None 1.5 59 12 Examples1621), 2.0 10% 43 Epichlorohydriu 4 Gross-linking 1. 5 2

CH2-CHCH2C1 17 Epichloroliydrin 15.5 do 1.5 4 0.2 2.0 8 0.8 Glyoidol15.5 substitutionuflnn 1.5 38 14 2.0 100 3a CH2CHCH2OH 19 a-Chlorohydrin10 d0 1.5 56 12 2.0 85 41 CICHz-CIIOH-CHzOH 2O 1,3-dichloroacet0ne 10Cross-linking 1.5 42 6 2.0 57 13 g ClCHzCCHz-C1 21 Chloroacetone 10Substitution 1.5 54 12 2.0 91 68 II CICHt-C-CHa The above examplesclearly show that when polyfunctional reagents are used to cross-linkcellulose prior to the papermaking refining operation, paper made fromthe resulting pulp is far more porous and absorbent than paper made frompulp which has not been cross-linked. In some instances the improvedpaper product made according to the present invention was as much assixty times more porous and fifteen times more absorbent thanconventional papers. Furthermore, this remarkable improvement inporosity and absorbency is obtained without any sacrifice in uniformityof formation and often in combination with an increase in uniformity offormation. It will also be noted from the foregoing examples that thepresent invention is applicable to cross-linking purified cotton linters(Examples 6 and 7) as well as raw cotton linters (for instance, seeExamples 2-5). In addition, the foregoing examples show that the presentinvention is applicable to employing a mixture of crosslinked lintersand noncross-linked linters, the improvements imparted to the resultingpaper made therefrom being proportional to the relative amount ofcross-linked linters used. The examples of Table 4 show that the presentinvention is not applicable to monofunctional reagents because suchreagents cause substitution in the cellulose molecule and givesubstantially no improvement in absorption and porosity of paper madefrom such substituted pulp (Examples 18, 19 and 21 compared with Example15), whereas polyfunctional reagents produce cross-linking and give asubstantial increase in absorbency and porosity in paper made from thecross-linked pulp (Examples 16, 17 and 20 compared with Example 15).Since the formation of both the cross-linking examples and thesubstitution examples in Table 4 above were substantially equal,formation did not help to distinguish cross-linking from substitutionand therefore was not reported in Table 4. Cross-linking Examples 16, 17and 20 in Table 4 are the same as 3, 4 and 5, respectively in Table 1.It is also apparent from the above examples that, by itself, treatmentpreparatory to cross-linking (i.e., caustic steeping and heating atreflux in acidic acetone) contributed substantially nothing to thedesired properties of the paper.

As mentioned hereinbefore, when formaldehyde or a polyaldehyde is thecross-linking reagent, we have found it necessary (1) that thecross-linking temperature not exceed about 60 C., (2) that the mineralacid used as cross-linking catalyst be a substantially nonoxidizingacid, and (3) that substantially all of the residual acid be removedfrom the cross-linked pulp before drying it. Thus, we have found thatcross-linking at a temperature much above 60 C. gives a paper which istoo brittle to be of any practical use (Example 22), whereascross-linking at 60 C. gives good results (Example 23). It is known inthis art that when using formaldehyde or a polyaldehyde one mustcross-link in the presence of an acid as cross-linking catalyst. Inaccordance with the present invention we have found that the acid mustbe a substantially nonoxidizing mineral acid. Thus, nitric acid is notoperable for the purposes of our invention even at 60 C.; and this istrue whether or not the residual acid is removed before drying the paper(Examples 24 and 25). Operable mineral acids include e.g., sulfuric,hydrochloric and phosphoric acids, and their performance has been foundto be about the same according to our invention. In addition, we havefound that most all of the acid remaining after cross-linking must beremoved from the pulp before drying (Examples 26 and 23). Examples 27and 28 show that nitric acid degradation of the pulp becomes even moresevere as the cross-linking reaction temperature exceeds 60 C.

The following Examples 22-29 further illustrate our invention,particularly as to the critical conditions discussed in the immediatelypreceding paragraph. Except as otherwise shown, these examples werecarried out under the same conditions used in Examples 1-21. The maindifferences used in carrying out Examples 22- 29 include the following:sulfite pulp instead of cotton linters, 4/ 1 ratio in the mixture ofcross-linked/noncrosslinked pulp, and one hour heating time instead of1.5 hours and 2 hours (the latter being due to the fact that 9 sulfitepulp requires less beating than linters since it hydrates considerablyfaster than linters).

The following procedure was used in Example 29 which is a controlexample. Satisfactory handsheets were obtained.

EXAMPLE 29 Table CONTROL A 500 gram sample of bleached sulfite pulp washeated at reflux (90 C.) for 1 hour in 20 parts'by volume of secondarybutyl alcohol containing 1% (by weight based on pulp) of sulfuric acid.After being thoroughly washed with water, the pulp was made intohandsheets and tested as described above. Additional data appear inTable 5 below.

EXAMPLE 22 Table 5 nonrininnnnxon The main purpose of this example wasto determine the efiect of increasing the crosslinking temperature above60 C. Although the handsheets could be removed from the screen, theywere too brittle to be of any practical use.

The conditions of Example 29 were repeated with 50 grams of formaldehydeadded to the acidic secondary butyl alcohol-sulfite pulp mixture beforerefluxing (90 C.). After being thoroughly washed with water, theresulting cross-linked pulp was made into handsheets and tested asdescribed above. Additional data appear in Table 5 below.

EXAMPLE 23 Table 5 FORHALDEHYDE The main purpose of this example was tosubstantially repeat Example 10, using sulfite pulp instead of cottonlinters. Satisfactory handsheets were obtained.

A 500 gram sample of bleached suliite pulp and 50 grams of formaldehydewere heated at reflux (60 C.) for 1 hour in 20 parts by volume ofacetone containing Table 5 FORMALDEHYDE 454 grams of bleached sulfitepulp was immersed in 40,000 grams commercial Eormalin solution solids)and 910 grams concentrated nitric acid solution. After 15 minutes, thesteeped pulp was pressed on a sintered glass filter to remove excesssolution. The resul ing pressed pulp (which contained about 30% byweight pulp on a dry basis) was heated at C. until substantially free ofwater. Additional data appear in Table 5 below.

EXAMPLES 27 AND 28 Table 5 FORlVLALDEHYDE Example 24 was repeated twice,except in Example 27 the pulp was dried at 90 C. and in Example 28 itwas dried at 120 C. The results were substantially the same as inExample 24 except the degree of pulp degradation by the acid wasnoticeably greater at the higher temperatures.

EXAMPLE 26 Table 5 FORMALDEHYDE Example 24 was repeated except sulfuricacid was used instead of nitric acid. Although the paper handsheets werestill unsatisfactorily pilly and brittle, the sulfuric acid degraded thepulp substantially less than the nitric acid.

Table 5 HANDSHEET PROPERTIES OF FORMALDEHYDE CROSS-LINKED SULFITE PULP[Etiect of cross-linking temperature, type acid, and whether acid isremoved before drying Example No 29 22 23 25 24 27 28 26 (control) AcidN None H S O4 H2304 HNO; HNOS HNO, HNO; H 504 Cross-linking temperature,G None 9 60 6O 60 90 120 60 Type process 1 None HT I-IT HT K K K K Airresistance (soc/1G0 cc./in. 550 113 110 120 Water absorption (sec) 96 7176 iaper handshects (formation and lie. i ity) Handshcots removableintact from wire screens Yes Yes Yes Yes No No N o No 1 HT=Presentinvention. K=Kantorowicz U.S. Patent 2,010,635. 2 Uniform and flexible.3 Uniform and brittle. Pilly and brittle.

Example 23 was repeated using nitric acid instead of sulfuric acid.Water was substituted for acetone as the reaction medium in order toavoid any possibility of introducing a hazard with an acetone-nitricacid mixture. The handsheets were uniform and brittle and thereforeunsatisfactory. The nitric acid degraded the pulp sub- In the aboveexamples (Examples 24, 27, 28 and 26 of Table 5) where the Kantorowicztype process (K) was used the cross-linking temperature and the dryingtemperature were the same because Kantorowicz effects crosslinking bydrying, whereas in the above examples (Examples 22, 23 and 25 of Table5) where the type process (HT) of the present invention was used, thecross linking temperature was as shown and the drying temperature was 60C. The paper handsheets prepared according to the Kantorowicz processwere so pilly and nonuniform that any values obtained for porosity andabsorbency would be meaningless. It will be noted from the examples inthe present application that cotton linters give somewhat betterporosity and absorbency than sulfite 11 pulp. This is to be expectedhowever because it is well known that sullite pulp hydrates more readilythan linters, so that with a given amount of beating sulfite pulp giveslower porosity and absorbency than linters.

As pointed out hereinbefore, when cross-linking with formaldehyde or apolyaidehyde it is well known that an acid catalyst must be used. Wehave found according to the present invention that this acid catalystmust be a substantially nonoxidizing mineral acid. We have also foundaccording to the present invention that this acid catalyst must beremoved from the cross-inked pul prior to drying the pulp irrespectiveof the acid used. When we do not remove the acid prior to drying thepulp we have found that the pulp is degraded during drying to such anextent that it results in pilly or nonuniform and also brittle paper.When nitric acid is used as the catalyst, there is still greaterdegradation of the pulp during cross-linking (than when a nonoxidizingmineral acid is used), even when we cross-link at 60 C. and remove thenitric acid prior to drying. This is because nitric acid is an oxidizingagent as well as a hydrolyzing agent under the conditions of the presentinvention.

Thus, it will be seen that the present invention resides in thediscovery that cross-linked cellulose pulp responds differently tomechanical beating than do prior art pulps used for paperrnaking, sothat paper made from the beaten cross-linked pulp has an entirelydifferent and unexpected and quite desirable combination of suchproperties as porosity, absorbency and uniformity of formation. Morespecifically, the present invention enables the manufacture of a sheetof paper with greatly increased porosity and absorbency and at the sametime with at least as good and often better uniformity of formation.Heretofore, no means has been found to obtain high porosity andabsorbency without a serious sacrifice in uniformity of formationbecause with conventional pulps the porosity and absorbency decrease invalue with mechanical beating, whereas the uniformity of formationincreases with increase in beating.

The paper product of this invention has a number of important uses,perhaps the two most important at present being filter papers and whatis known in the art as absorbency papers. Filter papers, of course, areso well understood that the term is self-explanatory. To mention a fewspecific applications, filter papers are used as oil filters, batteryplate separators, air filters and filters as used in the chemicallaboratory. Absorbency papers include a wide and varied field ofultimate uses, such as e.g., electrical papers, saturation papers andimpregnation or paper plastics.

The various sources of cellulosic materials commonly used in themanufacture of paper are applicable in the process of the presentinvention. These include, e.g. cotton linters, wood pulp and the like.

The present application is a continuation-in-part of our copendingapplication Serial No. 724,817, filed March 31, 1958, and now abandoned,entitled Paper Manufacture.

As many apparent and widely ditferent embodiments of this invention maybe made without departing from the spirit and scope thereof, it is to beunderstood that we do not limit ourselves to the specific embodimentsthereof except as defined in the appended claims.

What we claim and desire to protect by Letters Patent is:

1. In the method for forming paper wherein an aqueous suspension of afibrous material is dewatered on a screen to form a matted web and saidmatted web is dried, the improvement which comprises employing as afibrous material a water insoluble and alkali insoluble crosslinkedcellulosic material, said cellulosic material having been cross-linkedby intra-fiber chemical bonding with a polyfunctional reagent beforebeing subjected to any refining operations in the papermaking process,the polyfunctional reagent being selected from the group consisting ofpolyhalides and polyepoxides, thereby obtaining a of substantiallyincreased porosity and absorbency and at the same time at least withoutany substantial sacrifice in uniformity of formation.

2. in the method for forming paper wherein an aqueous suspension of afibrous material is dewatered on a screen to form a matted web and saidmatted web is dried, the improvement which comprises employing as afibrous material a water insoluble and alkali insoluble cross-linkedcellulosic material, said cellulosic material having been cross-linkedby intra-fiber chemical bonding with a polyfunctional reagent beforebeing subjected to any refining operations in the papermaking process,the polyfunctional reagent being selected from the group consisting offormaldehyde and polyaldehydes, the crosslinking being effected at atemperature not in excess of about 60 C. in the presence of asubstantially nonoxidizing mineral acid and substantially all of saidacid which remains after said crosslinking being removed from saidcellulosic material before drying same, thereby obtaining a paper ofsubstantially increased porosity and absorbency and at the same time atleast without any substantial sacrifice in uniformity of formation.

3. The process of claim 1, wherein the cross-linking is effected byreacting the cellulosic material with a polyhalide.

4. The process of claim 1, wherein the cross-linking is effected byreacting the cellulosic material with a polyepoxide.

5. The process of claim 2, wherein the cross-linking is effected byreacting the cellulosic material with a polyaldehyde.

6. The process of claim 2, wherein the cross-linking is effected byreacting the cellulosic material with formaldehyde.

7. The process of claim 1, wherein the fibrous material employed inmaking the improved paper comprises a mixture of cross-linked cellulosicmaterial and noncrosslinked cellulosic material.

8. The process of claim 2, wherein the fibrous material employed inmaking the improved paper comprises a mixture of cross-linked cellulosicmaterial and noncrosslinked cellulosic material.

9. The paper product of claim 1.

10. The paper product of claim 2.

References in the file of this patent UNITED STATES PATENTS 2,010,635Kantorowicz Aug. 6, 1935 2,623,042 Vaughan Dec. 23, 1952 2,752,269 CondoJune 26, 1956 2,794,736 Cohen June 4, 1957 2,985,501 Gagarine May 23,1961 OTHER REFERENCES Mussen: TAPPI, pages 85, 86, and 87, Aug. 20,1942.

1. IN THE METHOD FOR FORMING PAPER WHEREIN AN AQUEOUS SUSPENSION OF AFIBROUS MATERIAL IS DEWATERED ON A SCREEN TO FORM A MATTED WEB AND SAIDMATTED WEB IS DRIED, THE IMPROVEMENT WHICH COMPRISES EMPLOYING AS AFIBROUS MATERIAL A WATER INSOLUBLE AND ALKALI INSOLUBLE CROSSLINKEDCELLULOSIC MATERIAL, SAID CELLULOSIC MATERIAL HAVING BEEN CROSS-LINKEDBY INTRA-FIBER CHEMICAL BONDING WITH A POLYFUNCTIONAL REAGENT BEFOREBEING SUBJECTED TO ANY REFINING OPERATION IN THE PAPERMAKING PROCESS,THE POLYFUNCTIONAL REAGENT BEING SELECTED FROM THE GROUP CONSISTING OFPOLYHALIDES AND POLYEPOXIDES, THEREBY OBTAINING A PAPER OF SUBSTANTIALLYINCREASED POROSITY AND ABSORBENCY AND AT THE SAME TIME AT LEAST WITHOUTANY SUBSTANTIAL SACRIFICE IN UNIFORMITY OF FORMATION.